The particular purpose of such devices is to detect the quality and quantity of a liquid or solid coating present on the front or rear windscreen of a motor vehicle and, in dependence thereon, automatically to influence a windscreen wiping system associated with the screen, i.e. to set said system into intermittent or continuous operation for the purpose of cleaning the screen.
A device for controlling a windscreen wiper motor has become known from DE 33 14 770 C2, which comprises an optoelectronic apparatus and a circuit arrangement having an I/V converter, a phase-selective rectifier and a low-pass filter. Said device is however so constructed that the beam intensity of the beam transmitter(s) of the optoelectronic apparatus is held constant and the conversion factor of the I/V converter is fixed. Thus, the circuit arrangement may indeed to some extent be satisfactorily automatically adapted to changing measuring conditions during wiper operation, such as, for example, dirt, rain or snow on the windscreens or response to temperature changes by the optoelectronic apparatus, in such a way that the drive motor receives meaningful control signals from the circuit arrangement, but only because said measuring conditions vary within a relatively narrow range. If, however, wide-ranging changes in measuring conditions occur, such as, for example, when different types of windscreen are in use, automatic production of meaningful control signals by the circuit arrangement is no longer possible, which ultimately results in the windscreen wiping system no longer functioning automatically in a satisfactory manner.